Input circuit for receiving a signal at an input on an integrated circuit

ABSTRACT

An input circuit for receiving a signal at an input on an integrated circuit, particularly a DRAM circuit, and for assessing the signal with respect to a reference voltage is provided. One embodiment provides a termination circuit for setting a termination voltage, wherein the termination circuit includes a first resistor and a second resistor connected in series between a high voltage potential and a low voltage potential, the termination voltage being tapped between the first and second resistors, a first voltage-dependent resistor element having a first resistance gradient connected in parallel with the first resistor and a second voltage-dependent resistor element having a second resistance gradient connected in parallel with the second resistor, wherein the resistance values of the first and second resistor elements are controlled by a control voltage to set the termination voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims foreign priority benefits under 35 U.S.C.§119 to co-pending German patent application number 103 15 527.9, filedApr. 4, 2003. This related patent application is herein incorporated byreference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to an input circuit for receiving signalsat an input on an integrated circuit, particularly at an input on a DRAMcircuit, and for assessing the received signal with respect to areference voltage.

[0004] 2. Description of the Related Art

[0005] Fast data links between integrated circuits involve the use ofterminated data lines in order to avoid signal reflections in an inputcircuit in the integrated circuit. In addition, the input circuitsprovide the data lines with a termination voltage around which thesignals transmitted on the data line fluctuate, ideally with the sameswing. The termination voltage is prescribed by the input circuit andspecifies a center voltage around which the signals to be transmittedmove.

[0006] The termination usually implemented in current DRAM standards inthe input circuits is integrated directly in the integrated circuit andcannot be manipulated from the outside. Normally, the terminationvoltage is generated by a voltage divider made up of two resistors.Process-related fluctuations or temperature fluctuations mean that thetermination voltage is not constant, as a result of which the signalsreceived via the input circuit move around a center voltage which cannotbe stipulated exactly and is prescribed by the termination circuit.

[0007] To assess the received signal, the respective potential level ofthe received signal is compared with a reference voltage which can beprescribed externally or can be set in the integrated circuit. Thereference voltage cannot be aligned for different input circuits,however, but rather is normally provided for all input circuits on ageneral basis. It is thus conceivable that the termination voltage andthe reference voltage will differ from one another, which means that thevoltage swings between the high level and the reference voltage and thevoltage swing between the low level and the reference voltage will be ofdifferent magnitudes. This may result in one of the high or low levelsnot being able to be identified reliably under all circumstances.Another drawback is that, with cyclic signals having a predeterminedduty ratio, any discrepancy between the termination voltage and thereference voltage can result in a change in the duty ratio of thereceived signal. This is the case particularly for radiofrequencysignals, which have a small edge gradient.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide aninput circuit in an integrated circuit for the purpose of receivingsignals and reliably assessing them with respect to a reference voltage.It is also an object of the present invention to provide a method forsetting a termination voltage.

[0009] In line with a first aspect of the present invention, an inputcircuit for receiving a signal at an input on an integrated circuit isprovided. The input circuit assesses the signal with respect to areference voltage. The input circuit has a termination circuit forsetting a termination voltage. The termination circuit comprises a firstresistor, connected in series between a high voltage potential and a lowvoltage potential, and a second resistor, the termination voltage beingable to be tapped off between the first and second resistors. The firstresistor has a first voltage-dependent resistor element, having a firstresistance gradient, connected in parallel with it and the secondresistor has a second voltage-dependent resistor element, having asecond resistance gradient, connected in parallel with it. Theresistance values of the first and second resistor elements can becontrolled by a control circuit, in order to use the control voltage toset the resistance values of the first and second resistor elements andhence the termination voltage. In this way, a suitable choice of controlvoltage allows the termination voltage to be set such that the receivedsignal is assessed more reliably. Process-related discrepancies betweenthe resistance values of the first and second resistors can thus becompensated for. The control voltage controls the voltage-dependentresistor elements in different ways on account of the differentresistance gradients, which means that the resistance ratio of theresistance values in the two branches of the voltage divider and hencethe voltage produced thereby are altered. In one embodiment, the firstand second resistance gradients have different arithmetic signs in orderto obtain as large an adjustment range for the termination voltage aspossible.

[0010] In integrated circuits, the first and/or the secondvoltage-dependent resistor element may be produced using a transistor.In particular, the resistance gradients with different arithmetic signsmean that a p-channel field effect transistor is suitable for the firstvoltage-dependent resistor element and an n-channel field effecttransistor is suitable for the second voltage-dependent resistorelement.

[0011] A control circuit may be provided for setting the control voltagewhich has a voltage generator circuit for producing a comparison voltageand a differential amplifier. The voltage generator circuit may be ofthe same physical design as the termination circuit, with the comparisonvoltage and the reference voltage being applied to inputs on thedifferential amplifier. The control voltage can be tapped off at anoutput on a differential amplifier and is applied to the control inputsof the first and second resistor elements in the voltage generatorcircuit and in the termination circuit. In this way, the terminationvoltage can be aligned exactly with the reference voltage, since thevoltage generator circuit is essentially subject to the sameprocess-related and temperature-related fluctuations as the terminationcircuits. The differential amplifier is used to generate the controlvoltage on the basis of the extent to which the reference voltage andthe comparison voltage differ from one another. By feeding back theoutput of the differential amplifier to the inputs of the voltagegenerator circuit, the control voltage is aligned such that the voltagegenerator circuit and the physically identical termination circuit eachgenerate a voltage which essentially matches the prescribed referencevoltage. In this way, the termination voltage and the reference voltagecan be placed at equal potentials, which means that received signalswhose high and low levels move around the termination voltage can beassessed in the best possible manner.

[0012] In one embodiment, the comparison voltage is applied to thenoninverting input, and the reference voltage is applied to theinverting input of the differential amplifier. The output of thedifferential amplifier may also be applied to a plurality of terminationcircuits, so that the termination circuits of a plurality of inputcircuits are actuated using the control voltage generated by the controlcircuit. The voltage level chosen for the reference voltage may be avalue which is approximately in the center between the high level andthe low level of the received signal. For receiving and assessing thesignal, a signal evaluation circuit may be provided for the purpose ofcomparing the received signal with the reference voltage and assigning asignal value on the basis of the result of the comparison.

[0013] In line with a further aspect of the present invention, a methodfor setting a termination voltage on a termination circuit is provided.The termination voltage is set in line with a control circuit, thecontrol voltage being chosen such that the termination voltageessentially corresponds to a prescribed reference voltage against whichthe received signal is detected. The method provides the advantage that,when the reference voltage and the termination voltage are provided byproducing them independently of one another, the termination voltage canbe aligned with the prescribed reference voltage. In this case, thecontrol voltage is ascertained on the basis of the reference voltage bycomparing the termination voltage and the reference voltage with oneanother.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] So that the manner in which the above recited features of thepresent invention can be understood in detail, a more particulardescription of the invention, briefly summarized above, may be had byreference to embodiments, some of which are illustrated in the appendeddrawings. It is to be noted, however, that the appended drawingsillustrate only typical embodiments of this invention and are thereforenot to be considered limiting of its scope, for the invention may admitto other equally effective embodiments.

[0015]FIG. 1 shows a conventional input circuit, based on the prior art;

[0016]FIG. 2 shows a possible signal profile for a termination voltageand a reference voltage; and

[0017]FIG. 3 is a circuit diagram illustrating a termination unit foruse in an input circuit according to one embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018]FIG. 1 shows a conventional input circuit 1 which is usually useda plurality of times in an integrated circuit (not shown). The inputcircuit has a termination unit 2 and an evaluation unit 3. Both thetermination unit 2 and the evaluation unit 3 are connected to aconnection pad 4 on the integrated circuit. The connection pad 4 can beused to receive, via a data line 5, external signals which are drivenonto the data line 5 by an external driver circuit 6 in line with asignal S which is to be transmitted.

[0019] The termination unit 2 provides a termination voltage V_(TT)which is obtained from the resistance ratio of a voltage divider 7having a first resistor 8 and a second resistor 9. The first and secondresistors 8, 9 are connected in series between a high voltage potentialV_(DD) and a low voltage potential GND, the termination voltage V_(TT)being tapped off between the first and second resistors 8, 9. Thetermination voltage V_(TT) is connected to the connection pad 4. Duringthe transmission of signals, the driver circuit 6 operates against theresistors 8, 9 in the termination unit 2.

[0020] The evaluation unit 3 compares the signal levels of the receivedsignal with a reference voltage V_(REF) which can be prescribedexternally or can be generated internally in the integrated circuit suchthat it can be adjusted. The evaluation unit 3 has a comparator 10 whichcompares the received signal with the reference voltage V_(REF) andassesses the signal on the basis of the arithmetic sign of thediscrepancy. The reference voltage V_(REF) is usually selected such thatit is essentially at a central potential, i.e., in the center betweenthe high potential and the low potential of the desired signal levels.Since the signal levels are determined by the termination voltage,however, the reference voltage V_(REF) may differ from the ideal centralpotential.

[0021] If the reference voltage V_(REF) differs from the centralpotential, then either the high level or the low level is identifiedless reliably. If the received signal is a clock signal, then edgegradients which are too shallow may result in a shift in the referencevoltage V_(REF) with respect to the central voltage involving a dutyratio which is different from that of the original signal being read atthe output of the comparator 10. Particularly in DRAM memory circuits,however, the duty ratio of prescribed clock signals is important to themanner of operation and is subject to stringent specifications.

[0022] Since the termination voltage V_(TT) is essentially stipulated bythe voltage divider 7, which means that the termination voltage V_(TT)cannot be changed by external influences, the reference voltage V_(REF)and the termination voltage V_(TT) can be balanced only with difficulty.

[0023]FIG. 2 shows the profile of a possible received signal withrespect to the reference voltage V_(REF) and the termination voltageV_(TT). As shown, a discrepancy between the reference voltage V_(REF)and V_(TT) at the lower potential reduces the voltage swings between thelow potential and V_(REF) and increases the voltage swings between thereference voltage V_(REF) and the high potential. In addition, theperiod of time for the high period TH is extended as compared with theperiod of time for a high period given identical termination andreference voltages. Likewise, the period of time in which the evaluationunit 3 identifies a low level is shorter than the period of time inwhich a low level is identified given identical termination andreference voltages. This also results in a shift in the duty ratio.

[0024] Particularly in DRAM circuits, because the timing of the clocksignal controls the internal sequences, the edges of the clock signalneed to be predetermined in line with a predetermined duty ratio. Thediscrepancy between reference voltage V_(REF) and termination voltageV_(TT) thus results in the duty ratio having been altered for thereceived cyclic signal, particularly if the edge gradient of the signalis small, e.g., on account of radiofrequency transmission. The smallerthe edge gradient and the greater the discrepancy between referencevoltage V_(REF) and termination voltage V_(TT), the greater, too, is thediscrepancy between the duty ratio and the duty ratio of the signaltransmitted to the input circuit.

[0025]FIG. 3 is a circuit diagram illustrating a termination unit foruse in an input circuit according to one embodiment of the invention.The termination unit includes a termination circuit 20 for producing thetermination voltage V_(TT) on the basis of a control signal TS andproviding the termination voltage V_(TT) on the input circuit'sconnection pad.

[0026] The termination circuit 20 has a third resistor 21 and a fourthresistor 22, which are connected in series between the high supplyvoltage potential V_(DD) and the low supply voltage potential GND. Afirst p-channel field effect transistor 23 is connected in parallel withthe third resistor 21, and a first n-channel field effect transistor 24connected in parallel with the fourth resistor 22. The control inputs ofthe first p-channel field effect transistor 23 and of the firstn-channel field effect transistor 24 are connected to the control signalTS. The two field effect transistors 23, 24 have opposite resistancegradients, which means that a rising voltage for the control signal TSresults in the resistance of the first p-channel field effect transistor23 increasing and the resistance of the first n-channel field effecttransistor 24 decreasing, and vice versa.

[0027] The control signal TS may be generated by a control unit 25. Thecontrol unit 25 includes a voltage generator circuit 26 and adifferential amplifier 27. The voltage generator circuit generates acomparison voltage V_(VGL) which is applied to a noninverting input onthe differential amplifier 27. The reference voltage V_(REF) is appliedto an inverting input on the differential amplifier 27.

[0028] Like the termination circuit 20, the voltage generator circuit 26is integrated in the integrated circuit and may be of the same physicaldesign, which means that the voltage generator circuit 26 and thetermination circuit 20 are subject to the same process influences andtemperature influences. The voltage generator circuit 26 includes afifth resistor 28 and a sixth resistor 29. The fifth and sixth resistors28, 29 are connected in series between the high supply voltage potentialV_(DD) and the low supply voltage potential GND. Between the fifthresistor and the sixth resistor 28, 29, the comparison voltage V_(VGL)is tapped off. A second p-channel field effect transistor 30 isconnected in parallel with the fifth resistor 28 and a second n-channelfield effect transistor 31 is connected in parallel with the sixthresistor 29. The control inputs of the second p-channel field effecttransistor 30 and of the second n-channel field effect transistor 31 areconnected to the control signal TS, which is tapped off at an output onthe differential amplifier 27.

[0029] Since the termination circuit 20 and the voltage generatorcircuit 26 are of the same physical design, are connected to the samevoltage potentials V_(DD), GND, and are actuated using the same controlsignal TS, the termination voltage V_(TT) which is provided on theconnection pad in the integrated circuit corresponds to the comparisonvoltage V_(VGL). The differential amplifier 27 has the task of makingthe comparison voltage V_(VGL) more like the reference voltage V_(REF)which is provided for the integrated circuit.

[0030] Accordingly, the control signal TS which can be tapped off at theoutput of the differential amplifier 27 has a value at which thecomparison voltage V_(VGL) and the reference voltage V_(REF) essentiallyhave the same voltage value. Since the termination circuit 20 and thevoltage generator circuit 26 are of the same physical design, thereference voltage V_(REF) and the termination voltage V_(TT) at theoutput of the termination circuit 20 thus also have the same voltagevalue. Hence, the termination voltage V_(TT) in an input circuit in anintegrated circuit can be aligned with an externally prescribedreference voltage V_(REF), which means that any discrepancy betweentermination voltage V_(TT) and reference voltage V_(REF) may be avoided.

[0031] The control process for the control signal TS takes place asfollows: if the comparison voltage V_(VGL) is larger than the referencevoltage V_(REF), then the differential amplifier 27 significantlyamplifies the differential voltage (according to its gradient) andapplies the amplified voltage signal to the control inputs of the secondp-channel field effect transistor 30 and the second n-channel fieldeffect transistor 31. The high positive voltage applied theretoincreases the resistance of the second p-channel field effect transistor30 and lowers the resistance of the second n-channel field effecttransistor 31. This lowers the comparison voltage V_(VGL).

[0032] In a similar manner, a low potential (close to a low supplyvoltage potential for the differential amplifier 27) may be applied as acontrol signal to the control inputs of the second field effecttransistors 30, 31. The result of this is that the resistance of thesecond p-channel field effect transistor 30 is low and the resistance ofthe second n-channel field effect transistor 31 is high. In this case,the comparison voltage V_(VGL) has been increased, which means that thefeedback loop is used to adjust the reference voltage V_(REF) and thecomparison voltage V_(VGL) to one another. If the comparison voltageV_(VGL) and the reference voltage V_(REF) are essentially identical,then the output of the differential amplifier 27 produces a voltagepotential which prescribes the appropriate operating points of the firstand second field effect transistors 23, 24, 30, 31, in order to obtainthe desired termination voltage V_(TT) and comparison voltage V_(VGL).

[0033] When a plurality of input circuits are produced in one integratedcircuit, it is possible for the control signal TS generated by thecontrol unit 25 to be provided for a plurality of termination circuits20 which are respectively provided in the input circuits in anintegrated circuit. This makes it possible to align the terminationvoltage V_(TT) while saving space, since only one control unit 25 needsto be provided for the entire integrated circuit. Since the controlsignal TS is essentially a constant voltage signal and the controlinputs of the field effect transistors 23, 24, 30, 31 do not represent aload for a constant voltage signal, the supply line lengths between thecontrol unit 25 and the termination circuits 20 are essentiallyinsignificant, which means that they do not lead to the expectation ofshifts between the termination voltages V_(TT) in the individualtermination circuits 20.

[0034] While the foregoing is directed to embodiments of the presentinvention, other and further embodiments of the invention may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. An input circuit for receiving a signal at aninput on an integrated circuit and for assessing the signal with respectto a reference voltage, comprising: a termination circuit for setting atermination voltage, wherein the termination circuit includes a firstresistor and a second resistor connected in series between a highvoltage potential and a low voltage potential, a first voltage-dependentresistor element having a first resistance gradient connected inparallel with the first resistor; and a second voltage-dependentresistor element having a second resistance gradient connected inparallel with the second resistor; wherein the termination voltage istapped between the first and second resistors; and wherein theresistance values of the first and second resistor elements arecontrolled by a control voltage to set the termination voltage.
 2. Theinput circuit of claim 1, wherein the first and second resistancegradients have different arithmetic signs.
 3. The input circuit of claim1, wherein the first and second voltage-dependent resistor elementscomprise transistors.
 4. The input circuit of claim 1, wherein the firstvoltage-dependent resistor element comprises a p-channel field effecttransistor and the second voltage-dependent resistor element comprisesan n-channel field effect transistor.
 5. The input circuit of claim 1,further comprising: a control circuit for producing the control voltage,the control circuit comprising a voltage generator circuit for producinga comparison voltage and a differential amplifier; wherein thecomparison voltage and the reference voltage are applied to inputs onthe differential amplifier; and wherein the control voltage is tappedfrom an output on the differential amplifier and applied to respectivecontrol inputs of the first and second resistor elements.
 6. The inputcircuit of claim 5, wherein the comparison voltage is applied to anoninverting input and the reference voltage is applied to an invertinginput of the differential amplifier.
 7. The input circuit of claim 5,wherein a plurality of termination circuits are provided which areactuated by the control voltage generated by the control circuit.
 8. Theinput circuit of claim 1, wherein the reference voltage is a voltagelevel which is approximately in the center between the high level andthe low level of the received signal.
 9. The input circuit of claim 5,further comprising a signal evaluation circuit for comparing thereceived signal with the reference voltage and assigning a signal value.10. The input circuit of claim 5, wherein the voltage generator circuitincludes a third resistor and a fourth resistor connected in seriesbetween the high voltage potential and the low voltage potential, athird voltage-dependent resistor element connected in parallel with thethird resistor, and a fourth voltage-dependent resistor elementconnected in parallel with the fourth resistor; wherein the comparisonvoltage is tapped from between the third and fourth resistors; andwherein the output of the differential amplifier is applied torespective control inputs of the third and fourth voltage-dependentresistor elements.
 11. A method for setting a termination voltage for aninput circuit, comprising: providing a termination circuit having afirst resistor and a second resistor connected in series between a highvoltage potential and a low voltage potential, a first voltage-dependentresistor element connected in parallel with the first resistor, and asecond voltage-dependent resistor element having a second resistancegradient connected in parallel with the second resistor; and controllingthe first and second voltage-dependent resistor elements to set thetermination voltage which is tapped between the first and secondresistors.
 12. The method of claim 11, wherein the first and secondvoltage-dependent resistor elements are controlled by a control voltagefrom a control circuit.
 13. The method of claim 12, wherein the controlvoltage is determined by comparing a reference voltage against acomparison voltage utilizing a differential amplifier.
 14. The method ofclaim 13, further comprising applying an output of the differentialamplifier to an input of a voltage generation circuit for generating thecomparison voltage.
 15. The method of claim 14 wherein the voltagegenerator circuit includes a third resistor and a fourth resistorconnected in series between the high voltage potential and the lowvoltage potential, a third voltage-dependent resistor element connectedin parallel with the third resistor, and a fourth voltage-dependentresistor element connected in parallel with the fourth resistor; whereinthe comparison voltage is tapped from between the third and fourthresistors; and wherein the output of the differential amplifier isapplied to respective control inputs of the third and fourthvoltage-dependent resistor elements.
 16. The method of claim 11, whereinthe control voltage sets the termination voltage at a value about thesame as the reference voltage.
 17. A circuit for receiving an inputsignal on an integrated circuit, comprising: a termination circuit meansfor setting a termination voltage, the termination circuit meanscomprising a first and a second controllable resistor means forproviding a first and a second resistances connected in series between ahigh voltage potential and a low voltage potential, wherein thetermination voltage is tapped from between the first and secondcontrollable resistor means, the termination voltage connected to aninput connection pad on the integrated circuit.
 18. The circuit of claim17, further comprising: a control circuit means for setting a controlvoltage utilized to control the first and second controllable resistormeans.
 19. The circuit of claim 18, wherein the control circuit meanscomprises a voltage generation means for generating a comparison voltageand an amplifier means for comparing a reference voltage against thecomparison voltage to determine the control voltage.
 20. The circuit ofclaim 19, wherein the control circuit means comprises a third and afourth controllable resistor means for providing a third and a fourthresistances connected in series between the high voltage potential andthe low voltage potential, wherein the comparison voltage is tapped frombetween the third and fourth controllable resistor means.